The kinetic expression and potential cellular source of tumor necrosis factor-alpha (TNF-alpha) in lipopolysaccharide-(LPS) induced acute lung inflammation was investigated using a rat model by Northern blot analysis, in situ hybridization and immunohistochemistry. LPS induced a polymorphonuclear leukocyte infiltrate in the lung that peaked between 6 and 24 hours. TNF-alpha messenger (m)RNA was strongly induced by LPS in whole lung tissues shown by Northern analysis. Both alveolar macrophages and polymorphonuclear leukocytes (PMNs), purified from bronchoalveolar lavage fluids of LPS-treated rats, were shown to express TNF-alpha mRNA by Northern analysis. However, PMNs displayed several times more TNF-alpha mRNA, relative to actin mRNA, than alveolar macrophages at 6 and 12 hours. By in situ hybridization, most of the cells positive for TNF-alpha mRNA at 6 and 12 hours seemed to be PMNs located within the tissue near bronchioles or vessels. By immunohistochemistry, TNF-alpha protein was localized mainly to alveolar macrophages at early times (1 to 3 hours) after LPS challenge, and thereafter, PMNs seemed to be the predominant source of TNF-alpha protein as more than 90% of total intraalveolar positive cells at 6 and 12 hours were PMN. Thus, our data provide the first in vivo evidence that PMNs can serve as a significant source of TNF-alpha at sites of acute inflammation.